The present invention relates generally to digital display systems, and more particularly to a method of reducing contouring artifacts in display systems that use a spatial light modulator display device.
One goal of a display system is to present the displayed image in a manner that reduces contouring artifacts. If a given number of intensity levels is evenly distributed from maximum to minimum intensity, the human eye will be more sensitive to intensity transitions at the lower levels The eye can perceive approximately a 2% change in luminance. If the signal driving the display device has sufficiently high resolution per pixel, the changes will be less than 2%. However, if it does not have sufficiently high resolution, the changes will be greater than 2% at lower levels and will result in a visual artifact known as contouring.
Some display systems use non-linear display devices, such as cathode ray tubes. In these systems, contouring is reduced by the non-linear nature of the display in combination with a gamma correction imposed on the analog signal that drives the display.
Display systems that use a linear display device, such as a spatial light modulator (SLM), drive the SLM with a digital signal having a given resolution per pixel. This display resolution is constrained by the bandwidth limitations of the SLM. For example, a typical SLM might be driven with a 8-bit per pixel signal. However, this display resolution is insufficient to avoid contouring. Because the SLM is a linear device it does not benefit from the gamma correction of non-linear devices, and in fact, the gamma correction is reversed prior to display.
In the case of SLMs, various digital processing techniques have been devised to reduce contouring without increasing the display resolution. One such method is to use an error diffusion circuit. The error diffusion circuit operates on a high resolution degamma signal, that is, a signal from which gamma correction previously imposed on the signal is all or partly reversed. The degamma process provides a signal having higher resolution than the display resolution. The less significant bits are considered xe2x80x9cerrorxe2x80x9d bits. For each pixel value, the circuit divides the error into components and distributes the components to neighboring pixels. However, the distribution of the error components to vertically neighboring pixels requires the use of line memories, which adds to the cost of the system.
One aspect of the invention is an error diffusion method of reducing contouring. This method is used for a display system having a linear display device, such as a spatial light modulator (SLM), with the SLM having a given per-pixel resolution (display resolution). The method operates on a high resolution signal, which carries the stream of pixel values of the image to be generated. The high resolution signal has an error component and a non-error (display resolution) component. The error component is comprised of the less significant bits of each pixel value, that is, the bits that are not part of the display resolution. For each pixel value, the error component of the previously processed pixel value is fed back and added to a random value. The sum is added to the high resolution value, and the resulting value is truncated to the display resolution. The truncated values comprise the signal that drives the SLM.
Other aspects of the invention are dithering methods of reducing contouring. Like the error diffusion method described in the preceding paragraph, the dithering methods operate on a high resolution signal that has an error component. One or more dither signals are added to the high resolution signal, and no feedback is used. In one embodiment, the dither signal is a stream of random values. When added to the pixel values of the high resolution signal, the random values from the random signal cause the non-error component of each pixel value to either shift up one bit, or to not shift up. The average frequency of this shifting is proportional to the error. In another embodiment, which reduces patterns that might appear as a result of the random value dither, the random value signal is added to a spatial-temporal dither signal and the sum is added to the high resolution signal. In both cases, the signal is truncated to provide the drive signal for the SLM.
An advantage of the invention is that contouring is reduced without increasing bandwidth demands on the SLM. The error diffusion method eliminates the line memory that was required in prior error diffusion circuitry. The result is a reduction in cost of the display system. The dithering methods further reduce costs because the dither circuitry has less logic circuitry than error diffusion circuitry.